There are currently three types of object_property_add_link() callers:

1. The link property may be set at any time.
2. The link property of a DeviceState instance may only be set before
   realize.
3. The link property may never be set, it is read-only.

Something similar can already be achieved with
object_property_add_str()'s set() argument.  Follow its example and add
a check() argument to object_property_add_link().

Also provide default check() functions for case #1 and #2.  Case #3 is
covered by passing a NULL function pointer.

Cc: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[AF: Tweaked documentation comment]
Signed-off-by: Andreas Färber <afaerber@suse.de>